Central bank based economies get closer to complete crash every year. Bailouts get more expensive. National debts accelerate. Taxes increase. The whole system is going toward a global economy crash. Cryptocurrencies, being a new kind of money, are not automatically subject to all the same laws that are bringing down those systems, and have other advantages, so as the existing economies crash, cryptocurrencies are the only way out. Even if Bitcoin crashes to pennies per bitcoin, its still a good investment (if you buy at that time) for that reason. Its going to grow because its not subject to many of the flaws of the existing system.
Cryptocurrencies would crash right along with fiat currencies. The Internet's infrastructure is not funded by bitcoins. It's funded by the same fiat currencies that some people deride as being essentially worthless. Until you can buy Cisco routers with bitcoins, pay your ISP bill in bitcoins, and pay the power bill in bitcoins, the cryptocurrency is very much subject to the same flaws in the system as fiat currency.
The only 'currencies' that would remain are those that have value as assets. Metal, (iron, copper, gold, silver), food, medicine, alcohol, shelter, weapons.